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Unrepaired DNA double-strand breaks can lead to apoptosis or tumorigenesis. In mammals double-strand breaks are repaired mainly by nonhomologous end-joining mediated by the DNA–PK complex. The core protein of this complex, DNA–PKcs, is a DNA-dependent serine/threonine kinase that phosphorylates protein targets as well as itself. Although the (auto)phosphorylation activity has been shown to be essential for repair of both random double-strand breaks and induced breaks at the immunoglobulin locus, the corresponding phosphatase has been elusive. In fact, to date, none of the putative phosphatases in DNA double-strand break repair has been identified. Here we show that protein phosphatase 5 interacts with DNA–PKcs and dephosphorylates with surprising specificity at least two functional sites. Cells with either hypo- or hyperphosphorylation of DNA–PKcs at these sites show increased radiation sensitivity.

Lactoferrin (LF), traditionally known as an iron-binding protein present in high concentrations in milk and various secretions, has emerged as a multifunctional protein involved in many aspects of the host defense against infection. Recently, LF has been shown to inhibit the growth of solid tumors and reduce experimental metastasis in mice, suggesting that LF also may play a role in the defense against tumorigenesis. Here we provide the sequence of the cDNA and promoter region, the chromosome assignment, and tissue expression pattern of a novel form of LF mRNA (delta LF). The sequence of delta LF mRNA is nearly identical to that of LF mRNA; however, at the 5' end, we find a novel sequence that replaces the N-terminal signal peptide sequence of LF mRNA. We map the delta LF mRNA to human chromosome 3 and find that both delta LF and LF sequences colocalize to the same cloned 90- to 150-kb genomic DNA fragment. We further show that the delta LF mRNA is the product of alternative splicing of the LF gene and likely is specified by use of an alternative promoter. Although we find delta LF mRNA at various levels in 20 of 20 adult and fetal human tissues, we do not find delta LF mRNA in any of 14 diverse tumor-derived cell lines.